$x$ delivery persons carry a total of $B$ bags of groceries up the stairs. They can each carry $y$ bags at a time, and they each make $2$ trips up the stairs. Write an equation that relates $x$, $B$, and $y$.
Answer: Let's look at the number of bags a single person carries. A single person carries $y$ bags every trip and does $2$ trips. So in total, the person carries $2y$ bags: $\begin{aligned} &\phantom{=}\left(y\,\dfrac{\text{bags}}{\text{trip}}\right)\left(2\,\text{trips}\right) \\\\ &=2y\,\dfrac{\text{bags}}{\cancel\text{trip}}\cdot\,\cancel\text{trips} \\\\ &=2y\,\text{bags} \end{aligned}$ Since there are $x$ delivery persons, the total number of bags is $2xy$ : $B=2xy$